A Phosphate-Dependent Requirement for Transcription Factors IPD3 and IPD3L During Arbuscular Mycorrhizal Symbiosis in .

During arbuscular mycorrhizal (AM) symbiosis, activation of a symbiosis signaling pathway induces gene expression necessary for accommodation of AM fungi. Here, we focus on pathway components ( and (), which are potential orthologs of , a transcriptional regulator essential for AM symbiosis. In the double mutant , hyphal entry through the epidermis and overall colonization levels are reduced relative to the wild type but fully developed arbuscules are present in the cortex. In comparison with the wild type, colonization of is acutely sensitive to higher phosphate levels in the growth medium, with a disproportionate decrease in epidermal penetration, overall colonization, and symbiotic gene expression. When constitutively expressed in , an autoactive DOES NOT MAKE INFECTIONS 3 induces the expression of transcriptional regulators and , providing a possible avenue for arbuscule development in the absence of IPD3 and IPD3L. An increased sensitivity of to GA suggests an involvement of DELLA. The data reveal partial redundancy in the symbiosis signaling pathway, which may ensure robust signaling in low-phosphorus environments, while IPD3 and IPD3L maintain signaling in higher-phosphorus environments. The latter may buffer the pathway from short-term variation in phosphorus levels encountered by roots during growth in heterogeneous soil environments.